This invention relates to a monolithically integrated semiconductor optical device which may comprise a semiconductor laser and a method of fabricating the same.
A type of semiconductor lasers called distributed feed-back laser diodes (DFB LD's) are promising as light sources for long-distance and high-capacity optical fiber communication systems because each DFB LD oscillates at a single wavelength by utilizing distributed feed-back and wavelength selectivity of a diffraction grating formed within the semiconductor structure of the LD. In principle a DFB LD can easily be integrated with other kinds of optical devices because, unlike the conventional Fabry-Perot type semiconductor lasers, DFB lasers do not need cleavage surfaces. Accordingly DFB LD's are condsidered to be promising also as light sources for integrated optical circuits.
An example of integration of a DFB LD with another optical device is the monolithically integrated DFB LD device reported by M. Yamaguchi et al. at a National Meeting of Institute of Electronics and Communication Engineers of Japan, Light-and-Radio Section, Autumn 1984, lecture papers part II, No. 272. This device employs a double-channel-planar-buried-heterostructure (DC-PBH) and is comprised of a first region formed with a diffraction grating in a light guide layer adjacent an active layer in the DC-PBH wafer and a second region wherein the light guide layer has no diffraction grating. Separate electrodes are formed on these two regions. The first region is excited to operate as a DFB laser and the second region is operated as a modulator. This integrated device was produced primarily for the purpose of suppressing chirping of emitted wavelength, which is liable to occur when a conventional DFB LD is subjected to direct modulation by reason of fluctuation of the carrier density in the active layer. The purpose is accomplished by operating the DFB laser region with a constant drive current to make stationary oscillation and by using the modulator region for amplitude modulation of the laser light.
However, the reported integrated optical device suffers from insufficiency of electrical insulation between the DFB laser region and the modulator region. That is, the resistance between the electrodes on the respective regions is as low as about 50.OMEGA. since in this device electrical insulation is provided merely by removing a p.sup.+ -InGaAsP cap layer in a region between the two electrodes. Therefore, a portion of the injection current for the modulator leaks into the DFB laser region and causes a change in the lasing wavelength of the DFB laser. For this reason the operation characteristics of the integrated device are not always satisfactory.
Importance of electrical insulation is not specific to the case of integrating a DFB LD with a modulator. In general, integration of a plurality of optical devices shoud be made so as to establish sufficient electrical insulation between the respective devices. For example, considering that in representative semiconductor optical devices such as laser diodes the terminal voltages are at the level of 1.5-2V, electric resistance of at least 2K .OMEGA. is necessary for limiting the amount of leak-in current to 1 mA or less. Despite enhancement of electrical insulation, good optical connections must be established between the integrated devices.